This kind of driving mechanism is described in U.S. Pat. No. 5,820,118 for example. This driving mechanism in the prior art is composed as follows.
A clamp rod is inserted into a housing, a middle high portion of the clamp rod is supported vertically movably on an upper wall of the housing, and a piston provided at a lower portion of the clamp rod is supported on a barrel portion of the housing vertically movably.
The above-mentioned prior art contains several problems.
That is, to move the piston smoothly, a relatively large fitting gap is required between an outer peripheral surface of the piston and an inner peripheral surface of the barrel portion of the housing. Therefore, the clamp rod tilts due to the fitting gap during clamping operation of the clamp rod. As a result, the clamp rod cannot be accurately guided.
In order to solve this problem, the present inventors considered the following structure prior to the present invention. That is, the clamp rod is provided with two upper and lower sliding portions axially at intervals, these sliding portions are accurately fitted into an upper end wall and a lower end wall of the housing respectively, and the piston is fixed to an intermediate portion between the two sliding portions of the clamp rod.
This example of the preceding invention is excellent in that the clamp rod is strongly supported on the housing at the upper and lower points, which enables the clamp rod to be guided accurately. However, any sliding gap cannot be omitted between each of the sliding portions and the housing, which contains a problem that the sliding gap could increase due to machining errors. Therefore, there has been room for improvement in guiding the clamp rod with higher accuracy.
It is a first object of the present invention to provide a new aligning drive mechanism that can guide an output member such as a clamp rod and the like with high accuracy.
And it is a second object of the present invention to put a positioning apparatus having the above-mentioned mechanism to practical use.